The control systems in modern commercial aircraft engines are designed to operate the engine in a safe manner throughout its operating envelope during its on-wing life. Conventional systems in which performance is regulated, such as these aircraft engines, typically achieve this objective using Min-Max control selection to prevent the system from exceeding safety or operational limits. In the case of aircraft engines, this is performed during transients due to throttle commands, for example. A transient is a commanded change in a system setpoint. The architecture of a conventional engine control system 100 is illustrated in FIG. 1.
Each of the physical and operational engine limits of concern has an associated unique regulator. Each of these regulators generates a desired fuel flow rate (Wf) that maintains the limit. The throttle is used as input to either an engine pressure ratio (EPR) setpoint regulator or a fan speed (Nf) setpoint regulator. The desired fuel flow from the setpoint regulator and each of the maximum limit regulators is input to a minimum (Min) selector. The output of this Min selector and each of the minimum limit regulator outputs are input to a maximum (Max) selector. Thus, the output from the Min-Max selector is the fuel flow rate command that ensures that none of the operational and safety limits will be violated. Definitions for the various acronyms are provided in Table 1 below.
The architecture of system 100, which is typical for today's commercial aircraft engine controllers, is inherently conservative. The engine limit regulators are typically designed as simple regulators (e.g., proportional-integral controllers) and they are each independently designed. The Min-Max architecture then ensures that the most conservative control input is chosen. While typically sufficient, in emergencies such as the Sioux City crash of United® flight 232 in 1989, this conservative design may prevent the engine from achieving faster response times that may be needed to provide adequate flight control.
A limit regulator can become active even when there is no immediate danger of reaching the limit. This results in a slower engine response than is actually necessary to maintain safe operation. The conservative limit protection approach of FIG. 1 prevents the engine from delivering the dynamic response that is actually achievable. Accordingly, improved limit regulation that yields faster transient responses may be beneficial.